Run and Walk Script not working

Hey everyone I have a walk/run script and I am getting an error which is stopping the script from working. I have looked through it and rewrote it and it still the same error the script I am using is:

 #pragma strict
 @script RequireComponent( AudioSource )
  
 var walkSounds : AudioClip[];
 var runSounds : AudioClip[];
  
 var walkAudioSpeed : float = 0.4;
 var runAudioSpeed : float = 0.2;
  
 private var walkAudioTimer : float = 0.0;
 private var runAudioTimer : float = 0.0;
  
 var isWalking : boolean = false;
 var isRunning : boolean = false;
  
 var walkSpeed: float = 8; // regular speed
 var runSpeed: float = 20; // run speed
  
 private var chCtrl: CharacterController;
 private var chMotor: CharacterMotor;
  
 function Start()
 {
     chCtrl = GetComponent(CharacterController);
     chMotor = GetComponent(CharacterMotor);
 }
  
 function Update()
 {
     SetSpeed();
      
     if ( chCtrl.isGrounded )
     {
         PlayFootsteps();
     }
     else
     {
         walkAudioTimer = 1000.0;
         runAudioTimer = 1000.0;
     }
 }
  
 function SetSpeed()
 {
     var speed = walkSpeed;
      
     if ( chCtrl.isGrounded && Input.GetKey("left shift") || Input.GetKey("right shift") )
     {
         speed = runSpeed;
     }
      
     chMotor.movement.maxForwardSpeed = speed;
 }
  
 function PlayFootsteps()
 {
     if ( Input.GetAxis( "Horizontal" ) || Input.GetAxis( "Vertical" ) )
     {
         if ( Input.GetKey( "left shift" ) || Input.GetKey( "right shift" ) )
         {
             // Running
             isWalking = false;
             isRunning = true;
         }
         else
         {
             // Walking
             isWalking = true;
             isRunning = false;
         }
     }
     else
     {
         // Stopped
         isWalking = false;
         isRunning = false;
     }
      
     // Play Audio
     if ( isWalking )
     {
         if ( walkAudioTimer > walkAudioSpeed )
         {
             audio.Stop();
             audio.clip = walkSounds[ Random.Range( 0, walkSounds.Length ) ];
             audio.Play();
             walkAudioTimer = 0.0;
         }
     }
     else if ( isRunning )
     {
         if ( runAudioTimer > runAudioSpeed )
         {
             audio.Stop();
             audio.clip = runSounds[ Random.Range( 0, runSounds.Length ) ];
             audio.Play();
             runAudioTimer = 0.0;
         }
     }
     else
     {
         audio.Stop();
     }
      
     // increment timers
     walkAudioTimer += Time.deltaTime;
     runAudioTimer += Time.deltaTime;
 }
 

If someone else could look over this maybe you can spot something I couldn't

Thanks

Edit Sorry Forgot the error code

NullReferenceException: Object reference not set to an instance of an object PlayerRunAndFootsteps.SetSpeed () (at Assets/Slender/Scripts/PlayerRunAndFootsteps.js:58) PlayerRunAndFootsteps.Update () (at Assets/Slender/Scripts/PlayerRunAndFootsteps.js:36)